The present invention relates to an image pickup apparatus and an image pickup apparatus system. More particularly, the present invention relates to a compact and low-cost image pickup apparatus that is capable of photographing high-quality images comparable to silver halide photographs and also capable of obtaining photographic effects equivalent to those which are available for silver halide photographs, e.g. utilization of blurring and cropping.
There are digital cameras designed for business use, e.g. printing, which are capable of obtaining images comparable in quality to photographs. In many of these digital cameras, the pixel pitch of the image pickup device is about 10 micrometers or more, and the number of pixels is 2 million or more. The size of the image pickup device is the APS size or larger in terms of the size of silver halide film. Many of these digital cameras use an optical system developed for conventional silver halide photography.
In these image pickup apparatuses, the correlation between the depth of field and the field angle is close to that in silver halide photography because they use an image pickup device having a format size close to that of silver halide film. Therefore, it is possible to obtain photographic effects equivalent to those which are available for silver halide photographs, e.g. utilization of blurring associated with the depth of field.
However, image pickup devices used in these image pickup apparatuses are relatively large in size. Therefore, there is a limit to the number of devices obtained from one wafer. Consequently, the cost cannot be reduced in excess of a certain limit.
In addition, because optical systems used for photography are those developed for the conventional silver halide photography, no consideration is given to problems inherent in apparatuses using an image pickup device, for example, the deterioration of image information due to the fact that the aperture portion has a fixed size and the integral of the amount of light entering the aperture portion becomes an image signal, and the effect of filters such as an optical low-pass filter for removing moire and an infrared cutoff filter for eliminating adverse effects of infrared rays on the image, and further the effect of the physical structure of the image pickup device on the image.
In the case of silver halide film, a color image is made up of layered portions that form C, M and Y color images, respectively. In the case of an electronic image pickup device, because a plurality of color images cannot separately be formed in one pixel, the common practice is to employ a method in which a plurality of photographing operations are carried out for each color, and color images thus obtained are combined together afterward, or a method in which a plurality of images of different colors are formed with a plurality of image pickup devices provided for a plurality of branched optical paths, or a method in which color filter elements are arrayed in a matrix over the image pickup surface of an image pickup device, and pixels are associated with the respective colors of the filter elements, and in which an intensity concerning a color other than the associated filter colors is obtained by a computation from surrounding pixels or other technique.
The first method has the problem that a moving object cannot be photographed because it takes time to take a photograph. The second method has the problem that the cost increases because it requires a plurality of image pickup devices and a member for branching the optical path, and the apparatus becomes unfavorably large in size because the optical path branching member is inserted between the optical system and the image pickup devices. Accordingly, to realize a compact and low-cost apparatus, it is desirable to employ the third method.
In this case, the matrix filter array requires a certain thickness because it is necessary to impart a specific spectral transmittance to each filter element. Moreover, it is impossible to infinitely narrow the spacing between the filter array and the photoelectric conversion surface because of the electrode structure and the method of forming the photoelectric conversion surface.
Accordingly, if light is incident on the image pickup device extremely obliquely, light passing through a filter element enters a photoelectric conversion surface region other than the one that is associated with this filter element and where the light should arrive. This makes it impossible to obtain color information accurately.
It is also conceivable to form an infrared cutoff filter and color filters from interference filters that utilize a dielectric multilayer film, thereby minimizing the thickness of the filters. However, such an interference filter involves the problems that the transmittance varies according to the incident angle of light because the interference filter performs its function on the basis of the optical path length of light passing through the filter.
Accordingly, in the case of an apparatus using a image pickup device, it is desirable that the angle at which light exiting from the optical system is incident on the image pickup device, particularly the incident angle of the principal ray, should be minimized.
However, there is no severe restriction on the exit angle of optical systems designed for silver halide photography. Therefore, the image pickup apparatus cannot use any of the optical systems lined up for silver halide photography.
Incidentally, the most effective way of reducing the production cost of image pickup devices is to reduce the device size so that an increased number of devices are obtained from one wafer.
Many of image pickup apparatus that have been commercially manufactured as relatively low-cost apparatus use image pickup devices of ⅓ to ⅔ inch size and with about 1 to 2.5 million pixels.
The pixel pitch of these image pickup devices is about 3 to 5 micrometers, which is markedly small in comparison to image pickup devices used in the above-described digital cameras for business use. It is known that if the number of pixels is further increased to attain image quality comparable to that of silver halide photographs by using an image pickup device of the above-described size, the pixel pitch also inevitably decreases, so that it becomes impossible to ignore the reduction in sensitivity of the image pickup device and the degradation of image quality caused by shot noise arising from the photon number fluctuation.
These image pickup apparatuses use an optical system optimally designed to exhibit the characteristics of the image pickup device used. Therefore, many of them are designed by giving full consideration to the above-described problems associated with the use of an image pickup device.
On the other hand, it is known that as the area of the image pickup device decreases, the focal length of the optical system becomes shorter, and the optical depth of field becomes greater, giving rise to problems which are unlikely to occur in the case of taking silver halide photographs using a 35-mm film or the like.
For example, even when it is intended to take a picture in which a person is imaged large and sharply in an unsharp background as in portrait photography, such a phenomenon may occur that the background does not become unsharp as intended, or dust or falling snow or the like between the subject and the photographer which the photographer does not recognize is clearly imaged undesirably by photography using an electronic flash.
To solve these problems, it is conceivable to use an optical system with a small F-number or to lengthen the focal length to thereby reduce the depth of field. However, with the former method, the optical system becomes large in size, and the number of lens elements constituting the optical system increases, causing the cost to increase. With the latter method, because the photographing field angle changes, there are cases where the photographer""s intended composition of a picture cannot be obtained. Accordingly, even if image quality comparable to that of silver halide photographs is attained in terms of resolution, color reproduction, etc., it is impossible to obtain photographic effects equivalent to those which are available for silver halide photographs.
Thus, it is impossible with the prior art to attain an image pickup apparatus capable of producing high-quality images comparable to silver halide photographs and providing photographic effects substantially equivalent to those which are available at the time of taking silver halide photographs and also capable of achieving reductions in both size and cost.
In view of the above-described problems associated with the prior art, an object of the present invention is to provide a compact and low-cost image pickup apparatus capable of providing high-quality images comparable to silver halide photographs and also capable of obtaining photographic effects equivalent to those which are available for silver halide photographs, e.g. utilization of blurring.
To attain the above-described object, the present invention provides an image pickup apparatus wherein an image of an object produced by an optical system is formed on an electronic image pickup device, thereby obtaining image information concerning the object. The image pickup device is a device having a matrix or mosaic color filter array provided over a photoelectric conversion surface, and the electronic image pickup device and the optical system satisfy the following conditions:
6.2/{square root over ( )}N less than P less than 21/{square root over ( )}Nxe2x80x83xe2x80x83(1)
3.8 less than N less than 20xe2x80x83xe2x80x83(2)
3.4xc3x97Pxe2x88x9225 less than N less than 20xe2x80x83xe2x80x83(3)
0 less than xcex8 less than 1.3xc3x97P+4xe2x80x83xe2x80x83(4)
where P is the pixel pitch (given in micrometers) of the image pickup device; N is the number of pixels (given in million) of the image pickup device; and xcex8 is the angle (xc2x0) at which the central ray of a light beam exiting from the optical system or a ray passing through the center of a stop is incident on the image pickup device at the maximum image height.
In this case, it is preferable to satisfy the following condition:
7.6/{square root over ( )}N less than P less than 18/{square root over ( )}Nxe2x80x83xe2x80x83(5)
It is also preferable to satisfy the following condition:
5.0 less than N less than 20xe2x80x83xe2x80x83(6)
It is also preferable to satisfy the following condition:
xcex8 less than P+4xe2x80x83xe2x80x83(7)
It is also preferable for the optical system to have a detachable structure.
In addition, the present invention provides an image pickup apparatus system including an electronic image pickup device that has a matrix or mosaic color filter array provided over a photoelectric conversion surface and that satisfies the above conditions (1) to (3). A plurality of optical systems are prepared to form an image of an object on the electronic image pickup device. The optical systems satisfy the above condition (4).
The reasons for adopting the above-described arrangements in the present invention, together with the functions thereof, will be described below.
Assuming that the standard angular resolution of the human eye is 1xe2x80x2 (corresponding to the visual acuity 1.0), because this is a value that provides the critical resolution, resolution at which a line-and-space pattern contained in an image can be recognized as a sharp image is considered to be lower (larger in angle) than the above-described angular resolution.
For example, if the solid angle of the resolution is assumed to be about 1.67xe2x80x2 (corresponding to the visual acuity 0.6), when an image is observed from a position 40 centimeters away from it, for example, it can be recognized as a sharp image down to a size of 0.194 millimeters.
When a print of A4 size (210 mmxc3x97297 mm) is observed from a position 40 centimeters away from it, the number of pixels required in the above-described case is about 1,100xc3x971,500=1,650,000 pixels. Considering that a margin of about 10 millimeters is left at each of the peripheral edges of the print, the number of pixels required is about 1,400,000 pixels.
Basically, numerical values hereinafter presented are determined on the assumption that a print of A4 size is observed from a position 40 centimeters away from it. When there is a linear relationship between the print size and the viewing distance, the number of pixels obtained does not depend on the print size. In general, however, as the print size decreases, the viewing distance tends to increase in comparison to the print size. Therefore, in the case of a small print size, e.g. A6 size (105 mmxc3x97149 mm), a smaller number of pixels than the presented number of pixels will suffice. In other words, in the present invention, an image that can be satisfactorily enlarged to a print size of the order of 8xc3x9710xe2x80x3 size or larger is regarded as a high-quality image comparable to silver halide photographs.
Incidentally, in the case of an image produced by using an image pickup device, pixel units that constitute the image are fixed naturally. Therefore, it is impossible to express an object having a spatial frequency higher than the pixel units. When a test chart having pattern portions with various spatial frequencies is photographed, patterns whose narrowest line width is equal to the pixel pitch are all observed entirely resolved. However, in the case of patterns having a spatial frequency higher than the pixel units, in which the line width is less than the pixel pitch and the narrowest line width is, for example, xc2xd of the pixel pitch, only low-frequency light and dark patterns, i.e. moire, can be observed.
In the case of an image with the number of pixels obtained by the above-described calculation, the smallest pixel units of the print can be clearly recognized by man with high contrast. Accordingly, when the results of the above-described test chart observation are extended to apply to an ordinary image, the image appears sharp up to a certain spatial frequency, but it rapidly becomes impossible to resolve image components having spatial frequencies higher than the smallest pixel units. Therefore, although the contrast of the image is high, the image is poor in gradation representation and appears rough.
To obtain an image of abundant gradation, it is necessary to represent spatial frequencies up to one close to the resolution limit for the human perception.
In this regard, we have already disclosed that it is necessary in order to obtain image quality comparable to that of silver halide photographs to use pixel units corresponding to a solid angle of at least 1.25xe2x80x2 (visual acuity 0.8), and about 2,500,000 pixels are needed when a margin of about 10 millimeters is left for A4 size.
However, the above numerical value corresponds to the full-size print, in which the whole photographed image is printed. If the ease of use comparable to that of silver halide photographs is taken into consideration, some cropping will be needed. To obtain the same image quality after cropping, the same number of pixels will be needed after cropping. For example, to effect cropping so as to double the size of an image, four times as many as the original number of pixels is needed because, in terms of area, one fourth of the image is to be printed over an area of the same size as the original image. However, it is well recognized that even in the case of silver halide photographs, the image quality is degraded by cropping. Therefore, there are a few cases where cropping is done to such an extent as to double the size of the original image. In addition, it is possible to permit some deterioration of image quality caused by image processing, e.g. interpolation. As a consequence of examining these facts together, we found that if the original image is made up of pixel units corresponding to a solid angle of the order of 1xe2x80x2, it is possible to effect cropping within the normal range without causing a problem.
When the solid angle of 1xe2x80x2 is converted into a number of pixels by a calculation similar to the above, it is about 3.8 million pixels when a margin of 10 millimeters is left at each of the peripheral edges of a print of A4 size.
The above-described condition is expressed by the lower limit of the following condition (2).
3.8 less than N less than 20xe2x80x83xe2x80x83(2)
where N is the number of pixels (given in million) of the image pickup device.
The upper limit of the condition (2), i.e. 20, is determined by taking into consideration the fact that as the number of pixels increases, the image reading speed and the recording speed to a recording medium become excessively low. The upper limit value of the condition (2) corresponds to the above-described solid angle of 0.0513xe2x80x2 (visual acuity 2.3), which is determined by taking into consideration the number of pixels necessary to effect cropping to double the size of the original image or more and also taking into consideration the fact that at the upper limit value, even if the image is enlarged to A3 or larger, for example, and thus the viewing distance becomes short in comparison to the print size, the observer cannot practically recognize deterioration of the image quality.
To produce an image pickup device having the above-described number of pixels, it is desirable to satisfy the following condition (1):
6.2/{square root over ( )}N less than P less than 21/{square root over ( )}Nxe2x80x83xe2x80x83(1)
where P is the pixel pitch (given in micrometers) of the image pickup device.
The upper limit of the condition (1), i.e. 21/{square root over ( )}N, is set by taking into consideration the cost of the image pickup apparatus. If P is not smaller than the upper limit value, the size of the image pickup device becomes large in order to satisfy the number of pixels defined by the condition (2). Consequently, the number of devices obtained from one wafer decreases, causing an increase in cost. If P is not larger than the lower limit value of the condition (1), i.e. 6.2/{square root over ( )}N, even if the number of pixels is larger than the lower limit value of the condition (1), the diagonal length of the image pickup range becomes less than 9 millimeters, and the focal length of the optical system becomes short. Accordingly, it is impossible to solve the above-described problem that the depth of field becomes unfavorably great.
It is generally known that the number of defective pixels with respect to the total number of pixels of an image pickup device is proportional to the area of the image pickup device. Accordingly, as the area of the image pickup device increases, the production yield reduces. Even if the yield is constant, because the cost of an image pickup device is determined by the number of devices obtained from one wafer, the production cost of the image pickup device is supposed to increase as the area of the image pickup device increases.
From the user""s standpoint, not the area of the image pickup device but the image quality is valuable. Accordingly, an image pickup apparatus whose cost is high in comparison to the number of pixels is unfavorable. In view of these circumstances, we examined the allowable number of pixels and area of the image pickup device and, as a result, found it desirable to satisfy the following condition:
3.4xc3x97Pxe2x88x9225 less than N less than 20xe2x80x83xe2x80x83(3)
If N is not larger than the lower limit value of the condition (3), i.e. 3.4xc3x97Pxe2x88x9225, the number of pixels becomes small despite the large area of the image pickup device. Consequently, the cost-performance lowers. The upper limit value is the same as the upper limit value of the condition (2).
As has been stated above, it is desirable for a compact and low-cost image pickup apparatus to use an image pickup device having color filter elements arrayed in a matrix over a photoelectric conversion surface to construct a color image. In this case, it is desirable from the viewpoint of accurately obtaining color information that the angle at which light exiting from the optical system is incident on the image pickup device, particularly the incident angle of the principal ray, should be minimized.
To widen the allowable range of the angle of incidence on the image pickup device, it is conceivable to employ a technique whereby a light-blocking portion is provided around each of photoelectric conversion surface regions associated with the filter elements to prevent light passing through a neighboring filter element from reaching the photoelectric conversion surface region associated with the filter element concerned. If the ratio of the light-blocking portion to the pixel pitch is increased, the allowable range of the angle of incidence on the image pickup device further widens.
However, if the condition (1) or (3) is satisfied to ensure image quality comparable to that of silver halide photographs at reduced cost and the ratio of the light-blocking portion to the pixel pitch is increased, the area of the photoelectric conversion surface reduces. Consequently, the sensitivity lowers, and the S/N ratio deteriorates.
Considering the contribution of the optical system and the image pickup device to the image quality and size and the cost comprehensively, it is desirable that the angle at which the principal ray from the optical system is incident on the image pickup device should satisfy the following condition:
0 less than xcex8 less than 1.3xc3x97P+4xe2x80x83xe2x80x83(4)
where xcex8 is the angle (xc2x0) at which the principal ray exiting from the optical system is incident on the image pickup device at the maximum image height.
If xcex8 is not smaller than the upper limit of the condition (4), i.e. 1.3xc3x97P+4, the rate at which light passing through a neighboring color filter element reaches the photoelectric conversion surface region associated with the filter element concerned increases unless the proportion of the above-described light-blocking portion is increased, and it becomes difficult to effect correction with a shading correction circuit.
As has been stated above, in a case where interference filters utilizing a dielectric multilayer film are used as color filters, an infrared cutoff filter, etc., spectral characteristics vary according to the incident angle of rays. Therefore, it is desirable that not only the principal ray but also a light beam converging on each point in the image circle should be in a state close to that in the center of the image plane.
Assuming that the present invention may be applied to an apparatus that does not reduce the-aperture to such an extent that light exiting from the optical system can be regarded as consisting substantially of only the principal ray, the incident angle xcex8 in the condition (4) is not always the incident angle of the principal ray but may be the incident angle of the central ray of the light beam.
To allow the image pickup apparatus according to the present invention to provide an image of higher quality, it is preferable to further reduce the pixel units of the image. In this case, it is preferable to satisfy the following condition:
5.0 less than N less than 20xe2x80x83xe2x80x83(6)
To achieve a further reduction in cost, it is preferable to further reduce the size of the image pickup device. To utilize the effect of blurring at the time of taking a photograph, it is preferable to further increase the size of the image pickup device. Accordingly, it is preferable to satisfy the following condition (5):
7.6/{square root over ( )}N less than P less than 18/{square root over ( )}Nxe2x80x83xe2x80x83(5)
To further improve the accuracy of color information, it is preferable to satisfy the following condition (7):
xcex8 less than P+4xe2x80x83xe2x80x83(7)
In the image pickup apparatus according to the present invention, the optical system may be integrated with the apparatus. However, to make the apparatus conformable to photographic conditions suitable for various subjects, it is preferable for the optical system to have a detachable structure. Designing an optical system having such a structure and optimized for photographic conditions makes it possible to provide an optimum arrangement in terms of both size and cost.
In this case, the image pickup apparatus according to the present invention may be arranged in the form of an image pickup system. The image pickup system preferably includes an electronic image pickup device having a matrix or mosaic color filter array provided over a photoelectric conversion surface and satisfying the above conditions (1) to (3). A plurality of optical systems prepared to form an image of an object on the electronic image pickup device preferably satisfy the above condition (4).
To achieve both a further reduction in cost of the image pickup device and a photographic effect utilizing blurring comparable to that used for silver halide photographs, it is preferable to satisfy the following condition:
10.4/{square root over ( )}N less than P less than 16/{square root over ( )}Nxe2x80x83xe2x80x83(8)
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.